The present invention relates to deep brain stimulation (DBS) systems to treat certain medical conditions and, more particularly, to an implantable device having an array of selectively operable electromagnetic microcoils to induce currents in surrounding brain tissue.
Over 50,000 Americans are diagnosed with Parkinson's disease each year, with more than half a million Americans affected at any given time. Conventional treatments include pharmaceutical agents that produce dopamine, a neurotransmitter, in an attempt to replenish the low levels found in the brains of those suffering from the disease. Approximately ten percent of people with Parkinson's disease initially treated with pharmaceutical agents have little to no response. Electrical stimulation of the brain presents an alternative treatment option.
Specifically, deep brain stimulation has been used to effectively treat the symptoms of Parkinson's disease including rigidity, slowed movements, tremors, and walking difficulties. DBS treatment involves the surgical implantation of an electrical stimulator, often referred to as an electrode, lead, or implant, in the basal ganglia. Depending on the observed symptoms and treatment plan, DBS implants may be used to provide unilateral or bilateral simulation in the subthalamic nucleus (STN) or in the globus pallidus internus (GPi).
Existing DBS systems include one or more implants having a limited number of electrodes, a programmable current or voltage pulse generator, a battery, and electrical leads. Electrical impulses are created by the pulse generator, directed to the implants via the electrical leads, and continuously delivered to the STN or GPi brain sites via the electrodes up to twenty four hours per day.
The technology associated with DBS systems has the potential to help people afflicted with other physical ailments shown to respond to electrical stimulation. For example, stimulation of the brain's motor cortical areas has been used to help ischemic stroke survivors regain partial use of a weakened hand or arm. Further, it has been suggested that cortical brain stimulation can be successfully used to treat epilepsy. Other neurological disorders may also be treated with stimulation outside of the brain.
In spite of the clinical and potential successes, existing deep brain stimulation systems have a number of drawbacks. One drawback is the poor spatial resolution of existing DBS implants. Conventional cylindrical DBS implants have a very limited number of electrodes per implant because of spatial requirements between electrodes to prevent electrophoresis. Because of these gaps between electrodes, electrical stimulation of the brain may not be optimized. Further, during placement and setup of a DBS implant, a large variability exists between the location and size of the stimulation area within the brain and the amount of current to be delivered. Although numerical tools have been developed to estimate the volume of tissue activated (VTA) by each electrode, each DBS implant must still be positioned and set up on a case-by-case basis.
A second drawback of existing DBS systems is the large size requirement for electrodes in order to limit the effects of high current densities and electrophoresis. One conventional DBS implant includes cylindrical electrodes with a radius of 0.5 millimeters and a length of 2.5 millimeters. In practice, the dimension of each electrode is roughly equal to the portion of the brain intended to be stimulated, thus limiting the flexibility for spatially selective stimulation of the brain.
A third drawback of existing DBS systems is the use of copper-containing electrical leads between the pulse generator and the electrodes. These leads are not compatible with magnetic resonance imaging (MRI) procedures and special precautions must be adhered to during MRI procedures. While copper is not a ferromagnetic material and thus, the electrodes do not move or become dislodged when subjected to strong magnetic fields, large electrical currents may nonetheless be induced resulting in thermal damage to the brain tissue. DBS implants with elongated configurations or that are electronically activated are particularly prone to having induced currents.
Efforts have been made to overcome these and other drawbacks of existing DBS implants. Micro- and nano-electrodes, for example, may overcome the poor spatial stimulation characteristics of existing DBS implants and deliver currents into targeted brain regions to provide a more accurate physiological localization and stimulation. However, these implants still use capacitive coupling to deliver an electric current and thus, do not overcome the problems associated with direct electrode-to-tissue contact.
Transcranial magnetic stimulation (TMS), overcomes the problems of direct electrode-to-tissue contact by utilizing a non-invasive treatment. TMS devices utilize Faraday's law of electromagnetic induction that a changing magnetic field can induce electric current to flow in any conductive structure, including human tissue. TMS devices operate by passing a brief electrical pulse through one or more electrically conductive coils positioned adjacent to the human skull. The coils produce magnetic fields at right angles from the coils, through the skull, and into the brain. The magnetic fields, in turn, induce electric fields in the brain tissue to stimulate the nerves.
Although non-invasive, TMS has its own drawbacks. Because the intensity of magnetic fields produced by TMS devices decreases very rapidly away from the coil, stimulating deep regions of the brain requires very strong magnetic fields. However, high intensity electric fields (induced by the strong magnetic fields) are known to cause epileptic seizures and other neurological problems. Further, the induced electric fields are not sufficiently focused and, as a result, generalized stimulation throughout the brain may occur. Still further, the amount of electric current used to drive such TMS coils is prohibitively large.
Therefore, it would be desirable to have an apparatus that provides effective, accurate, and safe deep brain stimulation.